Dramatic advances in next-generation communication systems have inspired portable and compact mobile terminals with increasing channel capacity and throughput. According to the well-known Shannon's theorem, to improve the channel capacity of a communication system, one method is to broaden the system bandwidth, which has been adopted in the third and fourth generation mobile terminals. Another method is to use the multiple-input-multiple-output (MIMO) technology. This technology uses multiple antennas at both transmitter and receiver to improve the channel capacity by several-fold. Therefore, compact and broadband multi-antenna systems are required for future high-capacity mobile terminals.
As the wireless devices are becoming smaller and thinner, multiple antennas in portable terminals have to be implemented in a limited volume of space, and therefore, the spacing between antennas is far less than half-wavelength. This limited spacing will not only increase spatial/pattern correlation but also lead to strong mutual coupling between antennas. High spatial correlation will result in correlated channels and decreased channel capacity, whereas strong mutual coupling reduces radiated power, and thus reduces signal-to-noise ratio and eventually the channel capacity. This issue has drawn a great attention to many world leading companies.
To maintain a compact size of multi-antenna systems and to minimize interference between antennas, effective decoupling techniques need to be developed.